The use of thin-film antennas has been gaining popularity in recent years. Thin-film antennas are generally formed by applying a thin layer of conductive material to sheets of plastic film such as polyester, and then patterning the resulting sheets to form the conductive surfaces of antennas. Alternatively, conductive material may also be deposited on plastic or other dielectric sheets in desired patterns to form the antennas with the use of well-known masking and deposition techniques.
One area where there has been increased interest in using such thin-film antennas is for window-mounted applications in motor vehicles, aircraft, and the like. Due to the increasing need for different modes of wireless communication, thin-film window antennas represent a desirable alternative to populating a vehicle or aircraft structure with mast or other non-conformal type antennas, which can detract from the aerodynamic and aesthetic appearance of the surface.
Of course, the transparency of window-mounted thin-film antennas is an important consideration. To be useful as an optically transparent antenna, it is desirable that an antenna's transmittance to visible light be no less than about 70%. There are trade-offs between the optical transparency and the conductivity (or surface resistance) of thin-films utilized to make such antennas. For example, copper films having a surface resistance of about 0.25 milliohms/square are commercially available, but their transparency is well below the desired level of 70%. Other commercially available thin-films formed from conductive materials such as indium tin oxide (ITO) or silver have acceptable transparencies (for example, AgHT™ silver type films have optical transparencies greater than 75%), but such films have surface resistances in the range of 4-8 ohms/square, which is several orders of magnitude greater than that of the above copper films, or conventional conductors used for antenna construction. When transparent thin-films having these higher surface resistances are used as the conductive surfaces for an antenna, the performance of the antenna is diminished. Antenna efficiency is reduced due to ohmic loss in the higher resistance films, and as a result, antenna gain can be reduced by as much as 3-6 dB, depending upon the type of antenna.
In the past, attempts have been made to improve the efficiency of transparent thin-film antennas by increasing the conductivity of the surface. This is typically accomplished by increasing the thickness or type of conductive material applied, or by placing relatively thick sheets of non-transparent highly conductive material on the antenna. In doing so, the antennas become non-transparent. Without knowing the exact nature of the currents flowing on the surface of the thin-film antenna, the size of the areas where conductivity is increased can be made too large, thereby unnecessarily obstructing the optical view through a transparent antenna, or if areas of high current flow are not recognized and made more conductive, the resulting antenna will have a lower efficiency that could have otherwise been achieved.
Therefore, a need exists for a reliable method for improving the efficiency of antennas having transparent thin-film conducting surfaces, without unnecessarily obstructing the optical view through such surfaces.